Histogram modification has been used for some time to enhance images. However, these techniques are normally done in the computer and not in real-time.
The histogram could be modified in real-time by first digitizing the signal and then performing a nonlinear transformation with a table look-up memory. This technique requires sampling and digitizing the input signal at the Nyquist rate or higher. For video signals, this means very high-speed, high-power, analog-to-digital (A/D) converters and memories. Also, the images produced with this technique can have noticeable contouring because of the discrete nature of the digitized signal amplitudes.
This invention relates to a circuit which shapes the amplitude histogram of an analog signal source in real time. The histogram is continually estimated and used to update the transfer function of a nonlinear amplifier. One intended use is for contrast enhancement of television images in real time.
With the new approach, the required nonlinearity is accomplished with analog circuitry and the contouring problem should not exist. Second, since the input video scenes are slowly varying, the histograms will remain relatively constant for fractions of seconds or longer. Therefore, the histogram can be estimated from samples of the analog input signal taken at rates much below the Nyquist rate. Thus, a small low-power A/D converter can be used. Also, since the transfer function need only be updated at most a few times per second, a microprocessor can be used to control the transfer function. The microprocessor can run at relatively slow speed, resulting in low-power dissipation.
The implementation described herein has the potential of being produced in a very small package, and at low cost compared to an all-digital approach as used in the prior art.